Fluidized bed combustion of alternative solid fuels

Experimental Thermal and Fluid Science 28 (2004) 691–699

www.elsevier.com/locate/etfs

Fluidized bed combustion of alternative solid fuels

Fabrizio Scala *, Riccardo Chirone

Istituto di Ricerche sulla Combustione––CNR, P.le V. Tecchio, 80, 80125 Napoli, Italy

Abstract

The fluidized bed combustion of a number of alternative fuels of practical interest has been analyzed by a combination of

experimental and modeling techniques. Solid fuels of widely different origin (biomass, agricultural, civil and industrial wastes) have

been considered in this work. A lab-scale experimental campaign was carried out to evaluate the comminution (fragmentation,

attrition) behavior of the fuels. Experimental results have been used as input parameters in a stationary one-dimensional model of

an atmospheric bubbling fluidized bed combustor, suitable for high-volatile solid fuels. Model results are presented for a typical set

of operating variables and discussed with respect to location of combustion of fixed carbon and volatile matter and heat release

profile in the combustor. Differences between the combustion behavior of the various fuels considered are highlighted.

� 2004 Elsevier Inc. All rights reserved.

Keywords: Fluidized bed combustion; Alternative fuel; Fragmentation; Attrition

1. Introduction

Fluidized bed (FB) combustion of alternative solid

fuels is becoming more and more attractive as a result of

the constantly increasing price of fossil fuels, the pres-

ence of high quantities of wastes to be disposed of andglobal warming issues. By alternative fuels we mean a

wide range of non-fossil solid materials, ranging from

biomass and peat to municipal, agricultural and indus-

trial wastes, that can be burned alone or in combination

with fossil fuels. FB technology is usually indicated to be

the best choice, or sometimes the only choice, to convert

alternative fuels to energy due to its fuel flexibility and

the possibility to achieve an efficient and clean opera-tion. Extensive experimental investigation has been

carried out to date on the feasibility and performance of

different alternative fuels FB combustion [1–3]. Even if a

great amount of operating data has been collected so

far, however, detailed comprehension of the basic

mechanisms taking place during conversion in FB of

these fuels is still under way. Alternative fuels are often

treated just like fossil fuels in spite of the great differ-ences and variability of chemical and physical proper-

ties. These fuels, in fact, usually have a higher moisture

* Corresponding author. Tel.: +39-081-768-2969; fax: +39-081-593-

6936.

E-mail address: [email protected] (F. Scala).

0894-1777/$ - see front matter � 2004 Elsevier Inc. All rights reserved.

doi:10.1016/j.expthermflusci.2003.12.005

and volatile content, a more porous and fragile struc-

ture, often anisotropic, a lower density and a higher

intrinsic reactivity.

The high volatile content of some alternative fuels

leads to longer devolatilization times and larger quan-

tities of volatiles evolved. As a consequence, a distinctivefeature of these fuels is the larger heat release associated

with homogeneous combustion of volatile matter. Fuel-

inert particles and oxygen-volatiles mixing/segregation

processes in the bed play a much larger role with the use

of high-volatile fuels [4,5]. Coarse fuel particles rapidly

rise to the bed surface under the action of the bubbles

and remain there until devolatilization ends and their

size is reduced by combustion. A direct consequence ofvolatiles bypass of the bed is that the postcombustion of

volatiles in the splashing region and/or in the freeboard

leads to significant local overheating with respect to the

bed [6,7]. The location of volatile matter combustion

significantly affects the heat release profiles along the

combustor. This issue is strictly connected to the

extension and location of heat exchange surfaces, to

the pathways to pollutants formation, to the reliabilityand safety of combustor operation.

Operation of FB combustors firing alternative solid

fuels is generally associated with higher combustion

efficiencies compared with coal, provided that residence

time of the fuel particles in the bed is long enough [6].

On the other hand, carbon fines are formed to a large

mail to: [email protected]

Nomenclature

d average size of char particles in the bed, mEc carbon elutriation rate, kg/s

k attrition constant (defined in Eq. (1)),

dimensionless

n1 primary fragmentation particle multiplica-

tion factor, dimensionless

n2 secondary fragmentation particle multiplica-tion factor, dimensionless

U superficial gas velocity, m/s

Umf minimum fluidization velocity, m/s

Wc carbon loading in the bed, kg

692 F. Scala, R. Chirone / Experimental Thermal and Fluid Science 28 (2004) 691–699

extent by attrition and fragmentation of coarse particles

[8]. Recently, the overall combustion–attrition behavior

of a number of alternative fuels was studied [9–14].

Attrition and fragmentation turned out to be far more

extensive in the case of high-volatile fuels. This feature

reflects the propensity of such fuels to give rise, upon

devolatilization, either to highly porous, friable chars or

even to a multitude of fragments of very small size.Combustion efficiency is directly determined by the rel-

ative extent of the combustion time scale and of the

residence time of char fines in the bed which, in turn, is

related to elutriation. Fixed carbon from high-volatile

fuels is converted via the generation of fines followed by

their postcombustion over their residence time in the

bed to an extent that can be comparable with that of

direct combustion of coarse particles [12,13].Scala and Salatino [15] recently presented a station-

ary one-dimensional model of an atmospheric bubbling

FB combustor, suitable for high-volatile solid fuels. The

model accounts for fuel particle fragmentation and

attrition in the bed, volatile matter segregation, and

turbulent postcombustion above the bed as well as

thermal feedback from the splashing region to the bed.

Results from calculations with a biomass fuel indicatedthat a significant fraction of the heat is released into the

splashing region of the combustor and this results in an

increase of the temperature in this region. Extensive bed

solids recirculation associated to solids ejection/falling

back due to bubbles bursting at bed surface promotes

thermal feedback from this region to the bed of as much

as 80–90% of the heat released by after-burning of vol-

atile matter and elutriated fines. Under some operatingconditions a significant fraction of the volatile matter

was predicted to burn in the upper freeboard.

In the present work the FB combustion performance

of a number of alternative fuels of practical interest has

been compared. Solid fuels of widely different origin

(biomass, agricultural, civil and industrial wastes) have

been considered. An experimental campaign was carried

out to evaluate the comminution parameters of thefuels. These have been used as input parameters of the

Scala and Salatino model for the FB combustion of

high-volatile fuels [15]. Model results are presented and

discussed with respect to location of fixed carbon and

volatile matter combustion and heat release profile in

the combustor.

2. Experimental

2.1. Experimental apparatus

The experiments were carried out in a stainless steel

atmospheric bubbling fluidized bed combustor 40 mm

ID and 1 m high. The fluidization gas distributor is a 2

mm thick perforated plate with 55 holes 0.5 mm in

diameter disposed in a triangular pitch. A 0.6 m high

stainless steel preheater is placed under the distributor.

The fluidization column and the preheating section are

heated by two semicylindrical 2.2 kW electric furnaces.The temperature of the bed, measured by means of a

chromel–alumel thermocouple placed 40 mm above

the distributor, is kept constant by a PID controller.

The freeboard is kept unlagged in order to mini-

mize fines postcombustion in this section. Gases are fed

to the column via two high-precision digital mass

flowmeters.

The same reactor is used in two different configura-tions for the experimental tests. In the first configuration

(Fig. 1A), used for particle fragmentation experiments,

the top section of the fluidization column is left open to

the atmosphere. A stainless steel circular basket can be

inserted from the top in order to retrieve fragmented

and unfragmented particles from the bed. The tolerance

between the column walls and the basket is limited to

reduce as much as possible the amount of carbon left inthe bed when pulling out the basket. The basket mesh is

of 0.8 mm, so that the bed material can pass through the

net openings.

In the second configuration (Fig. 1B), used for fines

elutriation rate experiments, the top flange of the fluid-

ization column is fitted to a two-exit brass head equip-

ped with a three-way valve. By operating this valve it is

possible to convey flue gases alternately to two remov-able filters made of sintered brass. Batches of material

can be fed to the bed via a hopper connected sideways to

the upper part of the freeboard. A paramagnetic ana-

lyzer and two NDIR analyzers are used for on-line

Fig. 1. Experimental apparatus: (A) basket equipped configuration and (B) two-exit head configuration. (A) (1) gas preheating section, (2) electrical

furnaces, (3) ceramic insulator, (4) gas distributor, (5) thermocouple, (6) fluidization column, (7) steel basket, (8) manometer, (9) digital mass

flowmeters, (10) air dehumidifier (silica gel). (B) (1) gas preheating section, (2) electrical furnaces, (3) ceramic insulator, (4) gas distributor, (5)

thermocouple, (6) fluidization column, (7) head with three-way valve, (8) sintered brass filters, (9) hopper, (10) SO2 scrubber, (11) stack, (12) cellulose

filter, (13) membrane pump, (14) gas analyzers, (15) personal computer, (16) manometer, (17) digital mass flowmeters, (18) air dehumidifier (silica

gel).

F. Scala, R. Chirone / Experimental Thermal and Fluid Science 28 (2004) 691–699 693

measurement of O2, CO and CO2 concentrations,

respectively, in the exhaust gases.

2.2. Materials

The bed material consisted of 180 g of silica sand,

corresponding to an unexpanded bed height of 0.1 m.Sand was double sieved in the nominal size range 300–

400 lm with Sauter mean diameter of 360 lm. Mini-

mum fluidizing velocity was 0.05 m/s at 850 �C.

Ten alternative fuels have been considered, together

with one sub-bituminous coal (South African) as a ref-

erence fuel. Properties of the fuels are reported in Table

Table 1

Properties of the fuels

SA coal Robinia

pseudoacacia

Pinus

radiata

Pine seed

shells

Olive

husk

Proximate analysis (as received), % (w/w)

Moisture 2.3 7.1 35.0 13.0 13.1

Volatile

matter

23.0 75.1 51.6 59.6 56.3

Fixed

carbon

60.2 16.6 13.2 26.6 26.2

Ash 14.5 1.2 0.2 0.8 4.4

Ultimate analysis (dry basis), % (w/w)

Carbon 67.2 43.9 47.3 48.5 51.8

Hydrogen 3.7 7.8 6.1 6.1 5.5

Nitrogen 1.2 0.02 0.2 0.2 1.2

Sulfur 0.6 – 0.05 <0.1 0.1

Chlorine – – – – –

Ash 14.8 1.5 0.4 0.9 5.1

Oxygen

(diff.)

12.5 46.8 46.0 44.3 36.3

LHV, kJ/kg 25,400 15,600 11,650 15,200 17,500

1. The fuels have different origin: two ligneous biomas-

ses (branches of Robinia pseudoacacia and chipped Pinus

radiata), two agricultural wastes (pine seed shells and

exhausted olive husk), two dried civil sludges (sewage

sludge and Biogran), two civil wastes (refuse derived

fuel––RDF and tyre derived fuel––TDF) and two

industrial wastes (pet coke and scrapped ebonite fromcar batteries). For all these fuels the particle size used in

the experiments was in the range 3.0–6.0 mm.

Fluidization gas consisted of compressed air from the

laboratory distribution line, technical grade nitrogen

from cylinders or a mixture of the two. Inlet oxygen

concentration in the combustor was varied between 0%

Sewage

sludge

Biogran RDF TDF Pet coke Ebonite

5.6 6.6 1.8 1.9 1.8 1.3

55.8 46.5 75.1 63.4 7.6 34.7

10.1 6.9 9.6 30.4 89.6 43.5

29.5 40.0 13.5 4.3 1.0 20.5

36.4 30.9 49.4 83.8 92.7 64.1

4.7 3.8 6.9 6.9 2.4 4.2

5.0 3.7 0.8 0.6 1.2 1.5

0.6 1.0 0.3 2.0 1.5 3.4

– 0.07 0.5 – – 1.3

31.3 42.8 13.7 4.4 1.0 20.8

22.0 17.8 28.4 2.3 1.2 4.7

9250 13,100 21,100 36,800 33,500 26,700


and 4.5% on volume basis. The fluidization superficial

velocity was 0.8 m/s at 850 �C.

2.3. Procedures

Two kinds of experimental tests were performed in

the fluidized bed combustor, namely particle fragmen-

tation experiments (primary and secondary) and fines

elutriation rate experiments. Whatever the type of test,

the reactor was charged with a bed of sand (180 g) and

heated to 850 �C prior to each experiment.

2.3.1. Particle fragmentation experiments

Experiments were performed using the basket

equipped configuration (Fig. 1A). For primary frag-

mentation experiments the bed of sand was fluidized

with nitrogen. During the run the basket rested on the

distributor. Following the procedure proposed by Chi-

rone et al. [16] experiments were carried out by injecting

single fuel particles into the bed from the top of the

column. After about 3 min, required to completely de-volatilize the fuel particle, the resulting char was re-

trieved by means of the basket in order to investigate the

number and size of the produced fragments. The

experiment was repeated with about 30 particles in order

to collect a statistically significant number of fragments.

Secondary fragmentation experiments were carried

out in the same apparatus configuration as for primary

fragmentation experiments. Fuel particles were first de-volatilized for 3 min by dropping them in the bed flu-

idized with nitrogen. Only particles that did not

fragment during pyrolysis were further used for sec-

ondary fragmentation experiments. Nitrogen–oxygen

mixtures with oxygen concentrations in the range 1–

4.5% (on volume basis) were used as inlet gas. Following

the procedure proposed by Chirone et al. [16], single

char particles were injected in the bed from the top ofthe column. At definite times the particle was retrieved

from the bed with the basket and the number and size of

the fragments were recorded. The run ended when no

more carbon was found in the basket, i.e. the fragments

were consumed by combustion (in oxidizing conditions)

and attrition down to a size smaller than the mesh

opening size. Also for secondary fragmentation experi-

ments each test was repeated several times in order tocollect a statistically significant number of fragments.

2.3.2. Fines elutriation rate experiments

Batches of fuel were injected into the bed kept slightly

above the minimum fluidization velocity in nitrogen.

When devolatilization was over superficial gas velocity

was increased to 0.8 m/s and nitrogen and air feeds were

adjusted to establish the required oxygen concentrationin the inlet fluidizing gas. Nitrogen–oxygen mixtures

with oxygen concentrations in the range 1–4.5% (on

volume basis) were used. Elutriated fines were collected

by means of the two-exit head (Fig. 1B) by letting the

flue gas flow alternately through sequences of filters (one

was in use while the previous one was replaced) for

definite periods of time. The difference between the

weights of the filters before and after operation, dividedby the time interval during which the filter was in

operation, gave the average fines elutriation rate relative

to that interval. Fines collected in the filters were further

analyzed to determine their fixed carbon content. This

procedure allowed time-resolved measurement of car-

bon elutriation rates.

3. Model description

Simulations have been carried out with a stationary

one-dimensional model of an atmospheric bubbling flu-

idized bed combustor, suitable for high-volatile solid

fuels [15]. The combustor is modeled as a series of three

reaction zones: the dense fluidized bed, the splashing

region, and the freeboard. Bed fluid dynamics is modeledaccording to the two-phase fluidization theory: gas in the

bubble phase is in plug flow, gas and solids in the

emulsion phase are well stirred. The splashing region

extends between the average bed surface level and the

maximum level reached by bed solids in their ejection/

fall-back trajectories associated with bubbles bursting.

This region has been considered to be perfectly mixed

because of the high turbulence that establishes above thebed surface. The model is based on material balances on

fixed carbon (present both as relatively large non-elu-

triable char particles and as fine char particles of elutri-

able size), volatile matter and oxygen in each combustor

section. The model takes into account phenomena that

assume particular importance with high-volatile fuels,

namely: fuel particle size change by fragmentation (pri-

mary and secondary); fines generation by attrition orpercolative fragmentation; volatile matter segregation/

combustion in the bed and turbulent postcombustion

above the bed. An energy balance on the splashing region

is set up to account for thermal feedback from the

splashing region to the bed associated to solids ejection/

falling back. Volatile matter and elutriated fines post-

combustion and radiative and convective heat fluxes to

the bed and the freeboard are considered in the balance.Either over-bed or submerged fuel feeding can be con-

sidered in the computations. Gaseous pollutants forma-

tion and emission have been neglected in the model.

The model was slightly modified for application to

the prediction of the fluidized bed combustion of TDF

[14]. In particular bypass of fines into the splashing re-

gion during devolatilization was considered. The

assumption was made that the fraction of fines burningin the bed was equal to the fractional volatile matter

burned in this zone, the remainder bypassing the bed

directly into the splashing region.


The model has been applied to the prediction of the

fluidized bed combustion performance of the alternative

fuels reported in Table 1. Dry fuel particles have been

considered. Fuel-related input parameters were evalu-

ated experimentally as reported in the following section.Values of the other model parameters can be found in

Scala and Salatino [15]. In particular, the bed inner

section was 1 m2, the combustor height 5 m, the bed

sand inventory 1400 kg. Intrinsic combustion kinetic

data for the different fuels were taken from Senneca [17].

4. Results and discussion

Table 2 reports the values of the comminution

parameters evaluated experimentally for the different

fuels. In the table n1 and n2 are the particle multiplica-

tion factors due to primary and secondary fragmenta-

tion, respectively. The particle multiplication factor

represents the number of coarse char particles generated

by primary (secondary) fragmentation from each initialcoarse fuel (char) particle. n1 is a measure of the ten-

dency of fuel particle to generate fragments upon dev-

olatilization, just after injection in the hot fluidized bed.

n2, instead, is a measure of the char particles fragmen-

tation tendency during the whole burn-off under the

action of particle–particle and particle–wall collisions.

The reported values of n2 are averages of the results

obtained under different oxygen concentrations (in therange 1–4.5%). Experimental results show that the dif-

ferent fuels have widely different behavior. Upon dev-

olatilization Robinia, olive husk, sewage sludge,

Biogran and pet coke did not show any appreciable

primary fragmentation, SA coal, ebonite, RDF, pine

seed shells and chipped Pinus were subject to moderate

fragmentation, while TDF completely shattered into a

multitude of small fines. On the other hand, secondaryfragmentation results showed that SA coal, pet coke,

ebonite, sewage sludge, Biogran and olive husk did not

undergo any fragmentation during burn-off, while Ro-

binia, RDF, pine seed shells and chipped Pinus were

subject to moderate fragmentation. Of course, as no

coarse char particles are present after devolatilization

for TDF secondary fragmentation has no sense for this

fuel.

Table 2

Experimental results

SA coal Robinia

pseudoacacia

Pinus

radiata

Pine seed

shells

Olive

husk

n1 4 1 4 1.5 1

n2 1 3 1.5 1.5 1

k 1.3E)7 naa naa 2.0E)8 1.0E)8

a Not applicable.

Table 2 also reports results of attrition experiments

for the fuels considered, in terms of the carbon attrition

constant k, defined by the equation [16]:

Ec ¼ kðU � UmfÞWc=d; ð1Þ

where Ec is the carbon elutriation rate, U the superficial

gas velocity, Umf the minimum fluidization velocity, Wc

the carbon loading in the bed and d the average size ofchar particles present in the bed. The values of k re-

ported are averages over the whole burn-off of the re-

sults obtained under different oxygen concentrations (in

the range 1–4.5%). Again, as no coarse char particles are

present after devolatilization for TDF attrition has no

sense for this fuel. Results show that olive husk, pine

seed shells and sewage sludge have a relatively low

attrition propensity, while SA coal, ebonite and pet cokehave a moderate attrition rate. Biogran particles gener-

ate a negligible quantity of carbon fines: this is because a

coherent ash shell forms around the unconverted carbon

core during burn-off, hindering detachment of carbon

from the particles by attrition.

Robinia, chipped Pinus and RDF have a much higher

attrition propensity than the other fuels as a conse-

quence of their high char porosity. However, because ofthe high reactivity of the generated char fines, these are

mostly burned during their residence time in the bed.

This results in the unsuitability of the attrition rate

experiments to measure the true fines generation rate. A

different indirect technique proposed by Salatino et al.

[12] was applied for these fuels to estimate the fines

generation rate. Results of the experiments showed that

fines generation is proportional to the carbon burningrate at the particle surface. Consequently, following

Scala and Salatino [15] the peripheral percolation

mechanism was applied in the model instead of the

attrition constant. According to this mechanism during

the course of combustion porosity at the particle surface

reaches a critical value and the cortical region collapses

producing a multitude of fines.

Table 3 reports the combustor operating variablesused for computations and model results for all the fuels

considered. Under the present operating conditions the

bed expanded height and the splashing region height

were calculated to be 1.6 and 0.3 m, respectively.

Calculations show that for all the fuels considered

Sewage

sludge

Biogran RDF TDF Pet coke Ebonite

1 1 2 >1000 1 3

1 1 4 – 1 1

7.0E)8 �0.0 naa – 3.2E)7 1.2E)6

Table 3

Operating variables and model results

Operating variables

Bed temperature, �C 850 Fuel feed mean size, mm 5.0

Superficial gas velocity, m/s 1.0 Fuel feeding Submerged

Excess air factor, – 1.2 Bed solids mean size, mm 0.6

Model results

Fuel SA coal Robinia

pseudoacacia

Pinus

radiata

Pine seed

shells

Olive

husk

Sewage

sludge

Biogran RDF TDF Pet

coke

Ebonite

Fuel feed

rate, g/s

30.4 45.9 47.3 45.5 41.8 54.1 67.5 38.4 22.0 22.9 30.5

Coarse char

mean size,

mm

2.5 3.2 2.3 3.2 4.0 4.0 4.0 2.3 – 4.0 2.8

Fraction of

volatiles

burned in

bed, %

79.6 31.6 31.6 24.8 64.2 64.2 64.2 20.4 57.5 83.9 77.9

Fraction of

the volatile

burned in

splashing

zone, %

20.4 51.2 51.6 56.7 30.1 31.6 33.1 59.8 38.8 16.1 22.1

Bed carbon loading, kg

Coarses 33.4 0.080 0.115 1.55 2.22 0.422 0.365 0.030 0.0 56.9 2.90

Fines 0.26 6.1E)4 0.062 9.5E)7 9.5E)6 2.5E)7 2.1E)5 4.2E)4 0.022 0.10 6.2E)3

Oxygen mole fraction, –

Bed (dense

phase)

0.015 0.104 0.100 0.064 0.057 0.105 0.111 0.156 0.129 0.010 0.046

Splashing

zone

0.037 0.053 0.054 0.048 0.039 0.040 0.039 0.064 0.092 0.036 0.035

Freeboard 0.037 0.035 0.037 0.035 0.035 0.035 0.035 0.035 0.058 0.036 0.035

Fixed carbon

combustion

efficiency in

bed, %

98.7 100 97.1 100 100 100 100 100 56.9 99.0 99.9

Total combustion

efficiency in bed,

%

95.2 58.1 59.3 70.6 84.0 73.3 72.0 33.5 57.4 96.7 90.8

Total combustion

efficiency in

combustor, %

99.0 100 98.8 100 100 100 100 100 86.9 99.3 99.9

Splashing zone

temperature, �C849 877 863 870 860 876 880 903 875 850 857

Heat generation rate, kW

Coarse char in

bed

578 154 166 473 429 197 170 62 0.0 527 422

Fine char in bed 47 147 153 0.3 0.2 0.2 0.02 66 146 122 35

Volatiles in bed 34 132 71 71 195 415 460 139 291 112 247

696

F.Scala,R.Chiro

ne/Experim

entalTherm

alandFluid

Scien

ce28(2004)691–699

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except TDF practically all the fixed carbon is converted

in the bed, leading to combustion efficiencies higher than

99%. However, for fuels with a high (Robinia, chipped

Pinus and RDF) or moderate (SA coal, ebonite and pet

coke) attrition propensity a relatively large amount offixed carbon conversion occurs via the generation of fine

particles by attrition, followed by fines postcombustion

during their residence time in the bed. A different sce-

nario, instead, applies to TDF: as a consequence of the

fact that this fuel undergoes extensive shattering upon

devolatilization (primary fragmentation), the carbon

loading as coarse particles in the bed is equal to zero and

only fines are present in the bed. A large fraction ofthese fines is released directly in the splashing region and

burns in this region or in the freeboard. A non-negligible

amount of fines leaves unconverted the combustor,

leading to a relatively low combustion efficiency. It is

useful to recall that the fixed carbon combustion effi-

ciency in the combustor is the result of the competition

between parallel fines combustion and elutriation/

entrainment processes [14].As regards the volatile matter fraction, for low/med-

ium-volatile fuels (SA coal, ebonite and pet coke) the

evolved volatiles are mainly burned in the bed, the rest

being converted in the splashing region (Table 3). As the

volatile matter content of the fuels increases, the amount

of volatiles bypassing the bed and burning in the

splashing region grows, leading to an increase of the

temperature of this region. Further, an increasingamount of volatiles escapes the splashing region and

burns in the freeboard section. As high as 17% of the

total heat release is predicted to be produced by volatiles

combustion in the freeboard for RDF under the present

operating conditions.

To better summarize the behavior of the different

fuels considered, Fig. 2 shows the distribution of the

heat release in the combustor, reported as the fraction ofthe total heat release ascribed to the combustion of a

phase (coarse char, fines, volatiles) in a combustor sec-

tion (bed, splashing zone, freeboard).

It is interesting to assess, at this point, the magnitude

of thermal fluxes that are established at steady state

between the different sections of the combustor. The

main scope is that of determining the extent to which

heat released in the splashing region is fed back to thebed (where heat recovery is more efficient) and the re-

lated temperature of the splashing region. Results of

computations are summarized in Table 3. For the high-

volatile fuels, the large importance of volatile matter

and/or fines postcombustion gives rise to a pronounced

overheating of the splashing region, as high as 53 �Cabove the bed temperature for RDF. However, large

thermal feedback to the bed prevents overheating frombeing even larger. About 40–80% of the heat released in

the splashing zone is fed back to the bed, depending on

the fuel. The contribution from the solids convection

SA coal Robinia pseudoacacia Pinus radiata

Pine seed shells Olive husk Sewage sludge

Biogran RDF TDF

Pet coke Ebonite

Coarse char in bedFines in bedVolatiles in bedFines in spl. zone+FreeboardVolatiles in spl. zoneVolatiles in Freeboard

Fig. 2. Distribution of the heat release in the combustor: fraction of the

total heat release ascribed to the combustion of a phase (coarse char,

fines, volatiles) in a combustor section (bed, splashing zone, free-

board).

SA coal Robinia pseudoacacia Pinus radiata

Pine seed shells Olive husk Sewage sludge

Biogran RDF TDF

Pet coke Ebonite

BedSl. zone (recirculated)

FreeboardSl. zone (lost)

Fig. 3. Distribution of the heat release in the combustor: fraction of the

total heat release ascribed to combustion in a combustor section (bed,

splashing zone, freeboard).


mechanism associated to particles ejection/fall-back is

by far dominant [15]. For SA coal and pet coke, in-

stead, the volatiles heat release in the splashing region

is so small that in this region a temperature slightly

below the bed one is established, leading to negative

values of the heat feedback efficiency to the bed. Fig. 3

shows the distribution of the heat release in the com-bustor, reported as the fraction of the total heat release

ascribed to combustion in a combustor section (bed,

splashing zone, freeboard). The heat production in the

splashing zone has been divided into two contributions:

the fraction fed back to the bed (recirculated) and the

fraction exchanged with the freeboard (lost). In this

way the total heat that must be extracted from the bed

can be calculated as the sum of the first two contri-butions (bed + recirculated), while the heat to be ex-

tracted from the freeboard is the sum of the other two

(freeboard + lost). Results show that for the low/med-

ium-volatile fuels (SA coal, ebonite and pet coke) more

than 95% of the heat must be extracted from the bed,

while this fraction decreases with high-volatile fuels to

values as low as 75% for RDF. It must be noted that

these results are relative to calculations carried out

assuming submerged feeding of the fuel in the bed.

Should the over-bed feeding option be preferred, vol-atiles postcombustion in the splashing region and in the

freeboard would be even more important, causing lar-

ger heat fractions to be extracted from the freeboard

section of the combustor.

5. Conclusions and recommendations

The FB combustion behavior of a number of alter-

native fuels of practical interest (biomass, agricultural,

civil and industrial wastes) has been investigated by a

combination of experimental and modeling techniques.

Lab-scale experiments were carried out to evaluate the

comminution (primary and secondary fragmentation,

attrition) behavior of the fuels. Experimental results

show that the different fuels have widely different com-minution behavior. It can be said that fuels with a higher


volatile matter content are subject to more extensive

comminution during their processing in the FB, as a

consequence of the more fragile structure of the char

particles after devolatilization.

Experimental results have been further worked out toproduce input parameters for a previously presented

stationary one-dimensional model of an atmospheric

bubbling fluidized bed combustor, suitable for high-

volatile solid fuels. Calculations show that for all the

fuels considered except TDF combustion efficiencies

higher than 99% are achieved. For the high-volatile fuels

a large amount of fixed carbon conversion occurs via the

generation of fine particles followed by their postcom-bustion in the bed. For TDF, instead, the carbon

loading as coarse particles in the bed is equal to zero and

only fines are present in the bed. A large fraction of

these fines burns in the splashing region and in the

freeboard. A non-negligible amount of fines leaves

unconverted the combustor, leading to a relatively low

combustion efficiency.

For all the high-volatile fuels considered a largeamount of volatile matter burns in the splashing region

and/or in the freeboard, leading to the overheating of

these regions. However, large thermal feedback to the

bed prevents overheating in the splashing region from

being even larger. Results show that for the low/med-

ium-volatile fuels most of the heat must be extracted

from the bed. The fraction of heat to be extracted above

the bed sensibly increases with high-volatile fuels.On the whole, the combination of the simple lab-scale

experimental techniques for comminution parameters

evaluation and of the FB model calculations represents

a powerful tool for the investigation of the FB com-

bustion performance of a wide range of alternative fuels.

Further comparison with pilot-scale FB steady com-

bustion data is, however, necessary to bring the analysis

to a predictive level.

Acknowledgements

ENEL Produzione Ricerca, Italy and I. Gulyurtlu,

INETI, Portugal, supplied respectively Robinia pseudo-

acacia, Pinus radiata and exhausted olive husk and

Biogran samples. The support of Mr. A. Cammarotaand Mr. S. De Gregorio in fluidized bed experiments is

gratefully acknowledged.

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Fluidized bed combustion of alternative solid fuels